This is a great effort, when you are talking about response time, it is ping response. Also, it would be a ping response on their primary site (which will usually give you 100% uptime). But, it depends on the response time of the shared Hosting server, where the websites are actually hosted - and what is the page load time. For instance, BlueHost on their primary website has 100% uptime and with a great response time. As soon as you get onto one of their servers, I have seen website page load time shoots up to over 30s.
You could think of the sites that share your server as your roommates; there's really not that much separating you from them. Sure, you can close the bedroom door, but they can still cause nightmares for you in the kitchen and the bathroom. In web hosting terms, all the sites share a single server's resources, so huge traffic spike on Site A may impact the neighboring sites' performances. It's even possible that another site could take down the shared server altogether, if it crashed hard enough.
A Content Distribution Network (CDN) is a network of servers distributed geographically. The purpose of a CDN is to deliver content on the internet much faster. If a user in Toronto is trying to access your website, the closest server to them will take care of this. But someone coming to your website from Sydney, Australia, will be catered to by a server closest to them. The main host server holds your website content, and all the other servers have a cached version each.